You can create a user group for each client (and create the page and group, and put the user in the group automatically, in a plugin attached to OnUserActivate)
Another way to go, which I find easier and faster, is to put the user ID of the "owner" in the 'createdon' field of the page (or have the user actually create the page using NewsPublisher - which sets it automatically).
Once that's done, your snippet can do this to see if the user "owns" the page:
$output = '';
if ($modx->user->get('id') === $modx->resource->get('createdon') {
$output .= $modx->getChunk('OwnerChunk');
}
return $output;
If the "Owner" content is specific to the page, you can put it in a TV called 'OwnerStuff' and do this:
$output .= $modx->resource->getTVValue('OwnerStuff');